An Active Matrix Organic Light Emitting Diode (AMOLED) may emit light because it is driven by a current generated when a driving TFT is in a saturation state. Different critical voltages would generate different driving currents when a same gray scale voltage is input, and this leads to an inconsistency in the currents. A uniformity in threshold voltages (Vth) of transistors during a process of Low-Temperature PolySilicon is very poor, and the Vth may further drift, and thus the uniformity in a conventional 2T1C pixel unit driving circuit is always poor.
The conventional 2T1C pixel unit driving circuit is as illustrated in FIG. 1, and this circuit only comprises two TFTs wherein a T1 functions as a switch and a DTFT is used for driving the pixel. Operations of the conventional 2T1C pixel unit driving circuit is also simple, and a control timing of the 2T1C pixel unit driving circuit is illustrated in FIG. 2. T1 is turned on when a scan level Vscan on a scan line Scan is low, and a gray scale voltage Vdata on a data line Data charges a capacitor C, while the T1 is turned off when the scan level Vscan is high, and the capacitor C is used for holding the gray scale voltage. Because VDD (an output voltage at a high level output terminal of a driving power supply) is high, the DTFT is in the saturation state, and a driving current of the OLED is I=K(Vsg−|Vth|)2=K(VDD−Vdata−|Vth|)2, wherein Vdata is a data voltage output from the data line Data, K is a constant related to a size of the transistor and a mobility of carriers, and the K would be determined once the size of the TFT and manufacture process are determined. The formula for the driving current in the 2T1C circuit comprises the Vth, therefore in such a driving scheme, brightness at different positions on a panel varies and the uniformity in the brightness is poor, and the reasons are in that, as described previously, the Vths of the TFTs at different positions on the panel would vary largely even if the TFTs are manufactured with the same process parameters, since a process of the LTPS is imperfect, such that the driving currents of the OLED under a same gray scale voltage vary.